<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        .box {
            width: 100px;
            height: 2000px;
            background: blue;
        }

        .search {
            width: 300px;
            height: 50px;
            background: red;
            position: absolute;
            left: 50%;
            transform: translateX(-50%);
            top: 100px;
        }
    </style>
</head>

<body>
    <div class="box"></div>
    <div class="search"></div>
    <script>
        var searchEle = document.querySelector(".search");
       
        window.onscroll = function () {
            var num = document.documentElement.scrollTop;
              console.log(num);
            if (num >= 100) {
                searchEle.style.position = "fixed";
                searchEle.style.top = "0px";
            }else{
                searchEle.style.position = "absolute";
                searchEle.style.top = "100px";
            }

        }





    </script>
</body>

</html>